Visual mental imagery is used as a way of accessing information in memory, as a method of reasoning, as an aid to comprehension, and in a variety of other ways. The proposal addresses two questions: what is computed by the neural networks that subserve visual mental imagery, and how do those networks differ in the two cerebral hemispheres? Visual mental imagery has recently been shown not to be a single, unitary ability. Rather, imagery specific cognitive operation (e.g., shifting the orientation of an imaged object). The proposed research is an attempt to characterize further the nature of the processing subsystems used in imagery and to explore their realization in the two hemispheres of the brain. These experiments are aimed at documenting selective deficits for specific types of imagery processing in one hemisphere, which should be interpretable as the ineffective operation of specific subsystems. The results obtained with split-brain patients will be compared to those obtained in age-and education-matched normal people using divided visual field presentation of stimuli.